Question: $ F = \left[\begin{array}{rrr}-1 & 0 & 4 \\ 3 & -1 & 2\end{array}\right]$ $ D = \left[\begin{array}{rr}4 & 4 \\ 5 & 2 \\ 3 & 4\end{array}\right]$ What is $ F D$ ?
Explanation: Because $ F$ has dimensions $(2\times3)$ and $ D$ has dimensions $(3\times2)$ , the answer matrix will have dimensions $(2\times2)$ $ F D = \left[\begin{array}{rrr}{-1} & {0} & {4} \\ {3} & {-1} & {2}\end{array}\right] \left[\begin{array}{rr}{4} & \color{#DF0030}{4} \\ {5} & \color{#DF0030}{2} \\ {3} & \color{#DF0030}{4}\end{array}\right] = \left[\begin{array}{rr}? & ? \\ ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ F$ , with the corresponding elements in column $j$ of the second matrix, $ D$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ F$ with the first element in ${\text{column }1}$ of $ D$ , then multiply the second element in ${\text{row }1}$ of $ F$ with the second element in ${\text{column }1}$ of $ D$ , and so on. Add the products together. $ \left[\begin{array}{rr}{-1}\cdot{4}+{0}\cdot{5}+{4}\cdot{3} & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ F$ with the corresponding elements in ${\text{column }1}$ of $ D$ and add the products together. $ \left[\begin{array}{rr}{-1}\cdot{4}+{0}\cdot{5}+{4}\cdot{3} & ? \\ {3}\cdot{4}+{-1}\cdot{5}+{2}\cdot{3} & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ F$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ D$ and add the products together. $ \left[\begin{array}{rr}{-1}\cdot{4}+{0}\cdot{5}+{4}\cdot{3} & {-1}\cdot\color{#DF0030}{4}+{0}\cdot\color{#DF0030}{2}+{4}\cdot\color{#DF0030}{4} \\ {3}\cdot{4}+{-1}\cdot{5}+{2}\cdot{3} & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rr}{-1}\cdot{4}+{0}\cdot{5}+{4}\cdot{3} & {-1}\cdot\color{#DF0030}{4}+{0}\cdot\color{#DF0030}{2}+{4}\cdot\color{#DF0030}{4} \\ {3}\cdot{4}+{-1}\cdot{5}+{2}\cdot{3} & {3}\cdot\color{#DF0030}{4}+{-1}\cdot\color{#DF0030}{2}+{2}\cdot\color{#DF0030}{4}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rr}8 & 12 \\ 13 & 18\end{array}\right] $